Efforts to legalize recreational use of marijuana appear to be on the verge of a major step forward this fall, as polls show voters in five states, including Massachusetts, leaning toward approval.

If that momentum holds through Election Day, the proportion of Americans living in states where they can legally smoke a joint or eat a pot brownie will surge from about 5 percent to roughly one quarter.

A clean sweep at the ballot box would be the culmination of a big swing in public opinion. A recent Pew Research Center poll showed 57 percent of Americans favor marijuana legalization, and just 37 percent are opposed — nearly the inverse of just a decade ago.

In Massachusetts, the debate over Question 4 has focused almost exclusively on the ramifications for public health, criminal justice, and the fabric of urban and suburban communities. But the votes here and in the other states with referendums — Arizona, California, Maine, and Nevada — could have national implications, too.

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A string of unexpected losses, particularly in left-leaning states like Massachusetts and California, would stall — if not stop — the legalization movement. A sweeping victory on Nov. 8, on the other hand, would ratchet up the pressure on the federal government to lift its decades-long prohibition.

Advocates say they don’t expect Congress to fully legalize the drug anytime soon. But they say Washington could move to give its formal, legal blessing to states that decide to make marijuana lawful on their own.

That would provide some stability for an industry that has been forced to rely on the forbearance of the Obama administration, which has declined to use federal resources to crack down on marijuana sales in states like Colorado and Washington, where the drug is legal.

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In recent years, baby boomers and members of Generation X have warmed to the idea of legalization. But Kiley, of Pew, said strong support among millennials, ages 18 to 35, has been a driving force in the overall change in public opinion.

What has moved millennials and others into the legalization camp is not entirely clear. But last year, when Pew asked supporters why they backed legalization, many said they did not believe marijuana is any more dangerous than alcohol or cigarettes. Others touted the drug’s perceived medicinal benefits.

Twenty-five states, including Massachusetts, have full medical marijuana programs in place. Voters in four states — Arkansas, Florida, Montana, and North Dakota — will decide medical marijuana referendums next month.

“I think we’re almost certainly at a tipping point,” said Keith Stroup, founder and legal counsel for NORML, a Washington-based group that has been lobbying for legalization since 1970. “If we should run the table, which is possible . . . there’s simply no turning back.”

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Kevin Sabet, president of Smart Approaches to Marijuana, a national group trying to block legalization, downplayed the idea that a big win for the marijuana movement this fall would mark a turning point in the debate.

“I have a long view on all this,” said Sabet, whose group has pushed more than $100,000 into the “No on 4” campaign in Massachusetts. “This effort is not going to be won or lost this November, or November 2018, or November four years from now.”

Congress, he said, has “no appetite” for big changes in marijuana policy, and that’s unlikely to change. And he predicted that public opinion will swing against legalization when its full effects become clear.

Sabet, who founded SAM with former Democratic congressman Patrick Kennedy and conservative commentator David Frum, makes no moral judgments about marijuana. And he said no one should be locked up for using the drug.

Instead, he warned that legalization would mean creating a multibillion-dollar marijuana industry on par with “Big Tobacco,” almost certain to market to children. “This isn’t about Cheech and Chong anymore,” he said. “These are people with Yale MBAs and Harvard law degrees. . . . It’s about money, and it’s high time people started to talk about this.”

Governor Charlie Baker and other leading opponents of marijuana legalization in Massachusetts have made similar arguments, as have opponents in California and other states.

But supporters say making marijuana legal — and taxing and regulating it — is better than leaving it underground.

“What is the worst kind of marijuana business to have in your state?” asked Graham Boyd, director of the pro-legalization New Approach political action committee. “It’s criminal cartels. And that’s what you’ve got now.”

New Approach, based in Washington, D.C., is funded by the family of the late insurance executive and marijuana activist Peter Lewis and other philanthropists, with some money from the marijuana industry. The group has given $4.3 million to the “Yes on 4” effort in Massachusetts, according to the latest campaign finance filings, making it the largest player in the campaign by far.

Boyd, whose organization is also active in the Maine legalization effort, said advocates have to take the concerns of centrist voters seriously if they’re going to be successful in ballot campaigns — emphasizing strict controls on marketing and sales to minors, for instance, and efforts to curb drugged driving.

The Massachusetts campaign, like others around the country, has been infused with that kind of rhetoric this year. And it seems to be working — or, at least, not standing in the way of the broad drift toward legalization.

After stumbling in the polls in the spring and summer, the legalization campaign appears to be leading. A WBUR survey released last week showed 55 percent of voters backing legalization and 40 percent opposed.

“I think that Massachusetts has long been a state that moves issues nationally,” said Jim Borghesani, a spokesman for the “Yes on 4” campaign. “We saw it happen it with gay marriage. . . . It wouldn’t surprise me in the least — if we succeed in Massachusetts, in conjunction with California — to see a movement that, again, will spread across the country.”